1. Field of the Invention
The present invention generally relates to deflection systems for charged particle beams and, more particularly, to systems for deflecting electron beams at high speed and positional accuracy, especially in electron beam lithography apparatus.
2. Description of the Prior Art
Many divergent types of devices have been known in the past which rely on the ability to deflect a charged particle beam at high speed and with positional precision. Examples of such devices include oscilloscopes, television and other cathode ray tube applications, electron microscopes, ink-jet printers and electron beam lithography apparatus. Among these devices, electron beam lithography has become widely used in the processes associated with the fabrication of integrated circuits, such as the fabrication of masks for making such integrated circuits as well as the devices, themselves, through a process known as direct write lithography. With increasing degrees of integration density of integrated circuits, the requirements for speed and positional precision of electron beam deflection in electron beam lithography has similarly increased.
Economies to be derived from fabrication of integrated circuits on large wafers and the desirability of minimizing wafer repositioning during the electron beam lithography process require that electron beam deflection arrangements have a large range. The large range is also necessary to permit the electron beam to reach registration marks, placed at the four corners of the exposure area, without repositioning of the wafer.
The requirement for large range imposes certain performance constraints on the electron beam deflection system which are inconsistent with high speed and positional accuracy. For instance, if electrostatic deflection is used to obtain high speed, large range cannot be obtained without resulting in unacceptable degrees of various aberrations of the image formed by the beam cross-section. As is known, aberrations will occur with both electrostatic and magnetic deflection and will generally vary with the deflection angle. However, for a given angle of deflection, magnetic deflection will produce much smaller degrees of aberration than electrostatic deflection for practical deflection sensitivities. If magnetic deflection is used to achieve reduced aberration, the driver circuits capable of providing a large deflection range characteristically exhibit a long settling time and cannot accommodate high speed without being subject to and/or causing unacceptable levels of noise. If bandwidth of the driver circuits is reduced to reduce noise and accommodate long settling times, beam relocation speed and, hence, throughput of exposed devices is reduced, raising the cost of the process. Even when major/minor or other multichannel deflection arrangements are used to separate the high speed and large range requirements in magnetic deflection arrangements, inductive interactions often result in positioning times for high accuracy (e.g. settling times) which are more characteristic of the low speed/large range major driver than the high speed minor driver. U.S. patent application Ser. No. 07/607,196, filed Oct. 31, 1990, by Charles A. Gaston et al. entitled Compensation of Mutual Inductance in Multi-Channel Deflection Yokes, assigned to the assignee of the present invention, provides a possible solution to the problems associated with inductive coupling of driver circuits in multi-channel arrangements. This solution is most easily implemented in a deflection arrangement having two levels of magnetic deflection. The disparity between required deflection range and required deflection resolution, however, tends to make the provision of more than two channels preferable. While the invention described in the above-noted application, which is hereby fully incorporated by reference, is applicable to systems of any number of channels, such an implementation becomes much more complex as the number of channels is increased and adjustment for exact inductive coupling compensation becomes difficult.
Moreover, in major/minor deflection arrangements or other multichannel arrangements providing a hierarchy among deflection speeds and ranges, a practical limit to the number of channels or levels in the hierarchy is rapidly reached. A particular and salient limit imposed on the number of levels is the physical length of the electron optical column and the ability to fit magnetic deflection coils or electrostatic deflection plates into that length at locations which are consistent with all other requirements, dimensions and elements of the electron optical design of the deflection arrangement. This latter physical constraint is particularly critical and limiting in electron beam deflection arrangements which provide a constant beam landing angle, preferably normal to the target plane, over the entire deflection field.
Telecentric deflection is particularly desirable in electron beam lithography since the exposure target may not have a perfectly planar exposure surface which is consistently normal to the incident beam. If the electron beam impinges on the target surface at an angle that varies with the amount of deflection, any surface irregularity or deviation from the design target plane will cause a dimensional distortion in the exposed pattern on the target. However, if such imperfections exist in the target, distortion of the exposure pattern will be minimized or eliminated if the beam is kept parallel to itself when under the influence of the deflection arrangements employed (e.g. the deflected beam path is kept parallel to the undeflected beam path). Therefore, it is desirable that the deflection arrangement provide that the electron beam always be exactly parallel to itself (e.g. parallel or at a consistent angle to the electron optical axis) over the entire deflection range. This property is commonly referred to as telecentricity and guarantees that the angle of incidence of the beam impingement on the target stays the same throughout the field for a planar surface. It should be noted that the property of telecentricity is distinct from the angle of impingement on the target being exactly or ideally normal to the target.
The provision of telecentricity can theoretically be achieved by placement of the major deflector at the front focal plane of the projection lens. However, this cannot be achieved in practice because of the finite physical sizes of the projection lens and the major deflector. Since a deflector cannot be made infinitely small, only a compromise solution can be achieved, at best. Therefore, providing a reasonably close approximation of telecentricity for a single coordinate deflection direction has required two electrostatic or magnetic deflectors at a distance from the focal plane of the projection lens: the first to provide deflection having a radial component with respect to the axis of electron beam optical column and a second, driven synchronously with the first, to remove the radial component and return the beam to a direction which appears to emerge from a point generally on the axis and positioned relative to the lens to optimize telecentricity. It must be realized that any deflection of the beam, whether by a deflection arrangement or a lens, presents a disruption of telecentricity which, while such disruption can be minimized, must be traded off with the other aberrations which must also be simultaneously minimized in the design.
To achieve telecentricity while minimizing deflection aberration, a variable axis immersion lens (VAIL) has been developed in which the axis of the projection lens can be maintained substantially in coincidence with the telecentrically deflected beam over the entire deflection range. This lens and its use in and electron beam deflection arrangement are disclosed in greater detail in U.S. Pat. No. 4,544,846, to Langner et al and the same is hereby fully incorporated by reference. In summary, however, it is sufficient to indicate that since the axis of the lens can be shifted to coincide with the telecentrically deflected beam, the beam always arrives at the lens along the axis of the lens and no deflection of the beam occurs. Since no deflection of the beam is produced by the lens, no disruption of telecentricity occurs and, in fact, the optimization of the beam imaging is separated from optimization of the beam deflection.
Application of such a lens to a two channel hierarchical deflection arrangement including both magnetic and electrostatic deflectors is disclosed in detail in U.S. Pat. No. 4,859,856, to Groves et al, which is also fully incorporated by reference. As disclosed therein, disturbance of telecentricity can be avoided while limiting electrostatic deflection to very small deflections at very high speed. Typically, increases in speed imply a reduction is positional accuracy. However positional accuracy can be maintained, consistent with high speed, by limiting the deflection range. Therefore, optimization of high speed and positional accuracy can best be achieved by dividing the deflection requirements between a plurality of hierarchical levels of deflection. The arrangement of Groves, by minimizing aberrations and avoiding additional blurring of the beam, maintains a beam edge acuity of 0.1 microns. This corresponds to a resolution (range of 10 mm divided by beam edge acuity) of 100,000 lines. (It should be realized, however, that this resolution figure is an indication of the number of pixels which can be resolved in the deflection field rather than the number of pattern features which can be produced.) By comparison, the resolution of a good electron microscope is typically about 1000 lines or two orders of magnitude less.
A principal function of the projection lens in the prior art is to bring the electron beam into focus at the target surface. However, the focus and astigmatism of the beam is also affected by deflection. As the beam is deflected over its deflection range, it undergoes effects which degrade its sharpness and quality. These are known collectively as deflection aberrations. Two such deflection aberrations which are generally correctable are focus and stigmation (astigmatism).
However, as with other aspects of any deflection system design, the effectiveness of focus and astigmatism correction depends, in large degree on the ability to ideally locate the coils used for such corrections. For instance, focus and astigmatism correction cannot be effectively achieved after deflection since the correction will then introduce a consequent error in the deflection.
The VAIL system, as applied to shaped beam lithography, suffers from the problem that the focus and stigmation correction elements cannot be placed at the best location for the performance of this function since no image of the source is formed above the collimator lens and deflectors. Any system must, of course be physically realizable and a need has existed for an electron optical deflection geometry and arrangement in which more than two levels of deflection hierarchy can be accommodated for reasons of speed and accuracy while maintaining telecentricity. Likewise, a further need exists concurrently with the above needs to avoid the interaction between deflection and correction of astigmatism and focus of the electron beam in order to fully assure that telecentric deflection will be maintained.